It's v difficult. Dh's sister doesn't believe in his depression either.
You tell them that you haven't told them before not to worry them.
You ask them to read or watch a good book/ interview with someone they respect as an expert.
You mention Churchill and his black dog frequently in your conversations, wondering if that was why he slept so strangely, boozed so often, was manic.
If they are labour supporters you mention Alistair Campbell's breakdown, him having to go to bed for three months and sleep, and his articles on it.
Don't know about Tory equivalents.
You point out with no eye contact over the washing up that he's not in denial, he will go to the doctor and things will get straightened out.
Over another set if washing up, you ask why they jumped to that conclusion? No blame, just asking.
You remain calm and assertive and factual and find the way in that they understand.
Not worth a row. They love you and were trying to protect you. Just the wrong thing to do. But not in their view malicious, I suspect.